Owing to the gradual miniaturization of electron instruments and to their constant improvement in recent years, resin-sealed semiconductor devices wherein semiconductor chips are sealed with a resin, such as an epoxy resin, have become widely used as thin and compact semiconductor packages and other devices of the surface-mounted type.
Resin-sealed conventional semiconductor devices generally have a semiconductor chip mounted on a chip-mounting component commonly referred to as a die pad, the electrodes of the semiconductor chip and the inner leads connected by thin gold wires, the components sealed with an epoxy-based or other resin and subsequently connected to conductive leads and coated with solder.
Japanese provisionally published Patent Applications 61-23348 and 63-54757 show techniques for manufacturing such resin-sealed semiconductor devices, including machining the undersurfaces of the tabs to relieve stress.
However, a serious disadvantage of conventional resin-sealed semiconductor devices similar to that described above is their poor thermal resistance during solder mounting of the package. Specifically, since vapor phase reflow soldering or infrared heating are generally used as the method for solder mounting of these semiconductor packages on printed circuit boards (PCB) and the like, not only the components to be joined by soldering but also the packages themselves are rapidly heated during such solder mounting and the water which is absorbed in the bulk of the resin and which has penetrated the semiconductor device during storage rapidly evaporates owing to said heating. The vapor generated diffuses along the interface either between the resin and the chip-mounting component or between the resin and the semiconductor chip. The components become detached from each other, and the vapor penetrates between the detached portions, increases its internal pressure, and causes cracks in the resin to form. Such crack formation becomes especially noticeable in compact and thin semiconductor devices. A resin, such as an epoxy resin, absorbs moisture during the period of storage which follows manufacture but precedes soldering. With formation of cracks, the resin-sealing effect is considerably reduced and the performance and life span of semiconductor devices are severely impaired.
Although the techniques proposed in the above Japanese provisionally published applications 61-23348 and 63-54757 are effective in preventing cracks from being formed on the lower surfaces of chip-mounting elements, these techniques do not prevent the formation of cracks on the upper and lateral surfaces, which cause wire breakage and other failures fatal to semiconductor devices. Additional disadvantages include a larger number of operations and higher production costs caused by the machining of the reverse surfaces of the elements.